The use of nanoparticles to make optical coatings is known. Various optical functions can be achieved with such coatings. For example, an anti-reflective coating can be achieved by forming a porous coating with an effective refractive index lower than that of the substrate (U.S. Pat. No. 2,432,484). Typically these anti-reflective systems comprise a binder and nanoparticles. For example, U.S. Pat. No. 6,921,578 describes a method for preparing anti-reflective coating systems in which a binder (e.g. tetraethylorthosilicate TEOS) is hydrolyzed in the presence of the nanoparticles using an acid catalyst. While these approaches can lead to a coating with anti-reflective properties they suffer from a number of draw backs. For example, it can be difficult to make such coatings on an industrial scale as it is not easy to make stable coating compositions that result in coatings with reproducible optical and mechanical properties. Also, in order to produce coatings with suitable optical properties (such as refractive index) it is necessary to implement high levels of porosity. This can be achieved by incorporating voids in the binder which leads to a loss of mechanical properties.
It has been suggested to utilize hollow or porous particles in coatings (see, for example, EP1674891, US2004058177, WO2005021259, WO2005059601, WO2006030720, and WO2006033456). This places the void inside the particle rather than in the binder network resulting in an anti-reflective coating with better mechanical stability. Despite the apparent advantages of these hollow particle systems there are several drawbacks. For example, prior art hollow particles have proven difficult to control in terms of size and morphology. This makes it difficult to produce coatings having appropriate and reproducible properties. Also, the manufacture of such particles can be problematic, especially on an industrial scale. Furthermore, in certain cases a monodispersed system is desired which can be difficult to obtain with prior art methods. In addition, the means by which the void is created in the particle is not always compatible with its use in optical coatings.
Surprisingly it has been found that batches of polymer core-metal oxide shell particles can be produced in a reproducible manner and used in optical coatings such as anti-reflective coatings. These coatings show a better mechanical stability than coatings with comparable filled nanoparticles at the same level of reflection.
According to one aspect of the present invention, there is provided a composition suitable for forming an optical coating, the composition comprising core-shell nanoparticles, wherein said nanoparticles comprise:                (a) core material comprising polymer; and        (b) shell material comprising metal oxide.        
In a particular embodiment of the invention there is provided a coating composition comprising core-shell nanoparticles, wherein said nanoparticles comprise:                (a) core material comprising polymer; and        (b) shell material comprising metal oxide, preferably silica,wherein said nanoparticles have a rod or worm-like morphology. In a specific embodiment, the polymer comprises a cationically stabilized co-polymer micelle—more preferably, a diblock or triblock copolymer. In another preferred embodiment, the polymer comprises a cationically stabilized latex.        
According to a further aspect of the present invention, there is provided an optical coating wherein the coating comprises core-shell nanoparticles wherein the particles comprise core material comprising polymer and shell material comprising metal oxide.
According to a further aspect of the present invention, there is provided a process for forming an optical coating, the process comprising:                (a) applying a composition comprising core-shell nanoparticles to a substrate; and        (b) curing said composition to strengthen the network and remove the polymer corewherein the core-shell nanoparticles comprise core material comprising polymer and shell material comprising metal oxide.        
According to a further aspect of the present invention, there is provided the use of core-shell nanoparticles for optical applications.
According to a further aspect of the present invention, there is provided a substrate at least partially coated with an optical coating composition comprising core-shell nanoparticles wherein the particles comprise core material comprising a polymer and shell material comprising metal oxide.
According to a further aspect of the present invention, there is provided an article comprising a substrate at least partially coated with an optical coating composition comprising core-shell nanoparticles wherein the particles comprise core material comprising a polymer and shell material comprising metal oxide.
According to a further aspect of the present invention, there is provided a thin-film coating comprising core-shell nanoparticles wherein the particles comprise core material comprising a polymer and shell material comprising metal oxide. As used herein, “thin-film” refers to coatings having an average thickness of 300 nm or less.